1. Field of the Invention
This invention relates to ferrous powder blends. In one aspect, the invention relates to machinable-grade, ferrous powder blends containing boron nitride while in another aspect, the invention relates to the use of a boron nitride powder comprising agglomerates of irregular-shaped submicron particles.
2. Description of the Prior Art
The making and using of ferrous powders are well known, and are described in considerable detail in Kirk-Othmer's Encyclopedia of Chemical Technology, Third Edition, Volume 19, at pages 28-62 Ferrous powders can be made by discharging molten iron metal from a furnace into a tundish where, after passing through refractory nozzles, the molten iron is subjected to granulation by horizontal water jets. The granulated iron is then dried and reduced to a powder, which is subsequently annealed to remove oxygen and carbon. A pure iron cake is recovered and then crushed back to a powder.
Ferrous powders have many applications, such as powder metallurgy (P/M) part fabrication, welding electrode coatings, flame cutting and scarfing. For P/M applications, the iron powder is often blended with selected additives such as lubricants, binders and alloying agents, A ferrous P/M part is formed by injecting iron or steel powder into a die cavity shaped to some specific configuration, applying pressure to form a compact, sintering the compact, and then finishing the sintered compact to the desired specifications.
Shaped P/M sintered compacts often require machining as one of the finishing steps to produce the desired P/M product. Where the P/M product is a mass-produced product (for which the P/M process is well-suited), then the speed and efficiency at which these P/M products can be produced will depend in part on the speed and efficiency of the machining step. The speed and efficiency of the machining step is in turn a function of, among other things, how easily the P/M sintered compact can be cut by the machining tool. Generally, the more difficulty in cutting the P/M sintered compact, the more energy required of the cutting tool, the shorter the life of the cutting tool, and the more time required to complete the machining step.
One of the methods for increasing the speed and efficiency of the machining step is to make a P/M sintered compact with a low coefficient of friction at the interface of the cutting tool and compact, and with improved chip formation properties. This can be accomplished by blending the ferrous powder with a friction-reducing agent, such as manganese sulfide or boron nitride, but these known agents for ferrous powders while operative, are subject to improvement. For example, while all agents are admixed with the ferrous powder prior to sintering, some either adversely affect the dimensional changes that are undergone by the compact during sintering, or generally reduce the strength properties of the sintered compact, or both. A significant effect on dimensional change can require a die change by the P/M part manufacturer, which is a costly step and thus be avoided if possible. Significant reduced strength properties of the sintered compact generally reduce its ultimate usefulness These undesirable effects are a function, at least in part, of the nature and amount of agent actually added to the ferrous powder, and identifying agents that can provide the desirable effects but at lower addition levels and cost is a continuing goal of P/M research.